Studies on cell-mediated immune response in mice infected with Mycobacterium leprae and Mycobacterium lepraemurium were carried out using lymphocyte transformation, delayed type hypersensitivity and other relevant immunologic parameters. These studies have indicated the following: 1. In the M. leprae infected mice, there was no early onset of delayed type hypersensitivity, (DTH) nor was there any "in vitro" stimulation of the lymphocytes in the pesence of homologous antigens, although such stimulation was evidenced when non-specific B and T cell mitogenic agents were employed. 2. The DTH in these animals was seen to the time when the organisms appeared to reach a plateau with a concurrent enhancement of stimulation with specific antigens of the lymphocytes. 3. In the M. lepraemurium infected mice, there was an early onset of DTH which appeared to decline as the infection progressed into a systemic stage. Lymphoycte transformation in these mice, showed a significant stimulation index with specific antigens as well as the B and T cell mitogenic agents. The stimulation with specific antigens appeared to correlate with the onset of DTH. In the later stages of infection, the stimulation with specific antigens once again paralleled the decline in DTH. Response to PHA, as opposed to LPS was not observed as progressive infection set in and a high rate of mortality was evidenced. BIBLIOGRAPHIC REFERENCES: Navalkar, R.G., R.R. Dalvi & P.J. Patel Antigenic Evaluation of Mycobacterium lepraemurium. J. Med. Microbiol. 8: 177 (1975). Navalkar, R.G., P.J. Patel & R.R. Dalvi. Immunological Studies on Leprosy: Separation and Evaluation of the Antigens of Mycobacterium leprae. J. Med. Microbiol. 8: 319; (1975).